Image forming apparatus and method

Title: Image forming apparatus and method.Abstract: A generation unit is configured to generate a plurality of pieces of chromatic color material data, for each color positioned at a surface of a dark portion in a color gamut that can be reproduced using a black color material and the plurality of chromatic color materials, in such a way as to set a number of specific colors of the plurality of chromatic color materials whose dots are arranged exclusively with dots of other chromatic color materials on a recording medium to be equal to or less than one color. ...

BACKGROUND OF THE INVENTION

The present invention relates to image forming processing for forming an image on a recording medium using a black color material and a plurality of chromatic color materials.

2. Description of the Related Art

The technique capable of realizing a highly developed color by enlarging the color gamut (i.e., color reproduction range) of a printer is conventionally known. As discussed in Japanese Patent Application Laid-Open No. 6-233126, it is conventionally known that particular color inks enlarging the color gamut of basic red, green, and blue colors are usable in addition to basic color inks of cyan, magenta, yellow, and black.

More specifically, it is conventionally known that a red color area of the color gamut can be enlarged by using a red ink that can reproduce a red color having a higher saturation value compared to a red color formed by overlapping a magenta dot and a yellow dot.

As discussed in Japanese Patent Application Laid-Open No. 2004-155181, it is conventionally known that a developed color formed by overlapping a yellow dot and a cyan dot in this order is different from a developed color formed in the opposite order (i.e., in order of a cyan dot and a yellow dot).

Further, it is conventionally known that the color gamut can be enlarged by designating the order of yellow and cyan in image formation when the color to be developed is reproducible only when the yellow and cyan dots are overlapped in this order. Similarly, the color gamut can be enlarged by designating the order of cyan and yellow in image formation when the color to be developed is reproducible only when the cyan and yellow dots are overlaps in this order.

As discussed in Japanese Patent Application Laid-Open No. 2005-88579, it is conventionally known that a developed color is deteriorative in properties if a red dot is overlapped with yellow and magenta dots compared to a color developed without overlapping these dots. Further, it is conventionally known that the color gamut can be enlarged by differentiating a dot layout pattern applied to red color from a dot layout pattern applied to other colors in such a way as to reduce the above-described overlapping probability, when quantized color material amount data is converted into binary data indicating formation/non-formation of a dot in relation to a predetermined dot layout pattern.

As a problem peculiar to a print product printed by a pigment inkjet printer that mainly uses pigments as coloring materials, the color gamut deteriorates at a dark portion (i.e., a low-lightness region). FIG. 1 schematically illustrates the shape of a color gamut relating to yellow hue of a print product printed by the pigment inkjet printer. In FIG. 1, the abscissa axis represents the magnitude of saturation C* and the ordinate axis represents the magnitude of lightness L* in a CIELCh color space, in which point A, point B, point C, and point D indicate colors adjacent to white, yellow, black, and color adjacent to black at the surface of the color gamut, respectively.

As illustrated in FIG. 1, the contour of the color gamut extending from yellow to black is greatly deformed inward at a dark portion (i.e., a low-lightness region), compared to a straight line connecting the point B (yellow) and the point C (black). Especially, in an area adjacent to black, the saturation does not change so largely even when the lightness changes greatly, as understood from the positional relationship between the point C and the point D. Therefore, the color gamut having a formed shape causes defective gradation (collapse) in color mapping.

In this respect, according to the techniques discussed in Japanese Patent Application Laid-Open No. 6-233126 and Japanese Patent Application Laid-Open No. 2005-88579, printers are required to have a complicated and enlarged structure due to newly added inks. Further, according to the technique discussed in Japanese Patent Application Laid-Open No. 2004-155181, the color gamut of intermediate lightness is enlarged in the hue extending from yellow to cyan via green. However, the color gamut of the dark portion (low-lightness region) extending from yellow to black cannot be enlarged.

SUMMARY

The present invention is directed to an image forming apparatus that can enlarge the color gamut of a low-lightness region without adding a new recording material.

According to an aspect of the present invention, an image forming apparatus is configured to form an image on a recording medium using a black color material and a plurality of chromatic color materials. The image forming apparatus includes a generation unit configured to generate black color material data for arranging a dot of the black color material on the recording medium and a plurality of pieces of chromatic color material data for arranging a dot of the plurality of chromatic color materials on the recording medium, based on input image data, and a formation unit configured to form an image on the recording medium using the black color material and the plurality of chromatic color materials based on the black color material data and the plurality of pieces of chromatic color material data. The generation unit is configured to generate the plurality of pieces of chromatic color material data, for each color positioned at a surface of a dark portion in a color gamut that can be reproduced using the black color material and the plurality of chromatic color materials, in such away as to set the number of specific colors of the plurality of chromatic color materials whose dots are arranged exclusively with dots of other chromatic color materials on the recording medium to be equal to or less than one color.

Further features and aspects of the present invention will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments, features, and aspects of the invention and, together with the description, serve to explain the principles of the invention.

FIG. 1 schematically illustrates the shape of a color gamut relating to yellow hue of a print product printed by a pigment inkjet printer.

FIGS. 19A and 19B schematically illustrate color separation tables of black-color line according to the first exemplary embodiment of the present invention.

FIG. 20 is a block diagram schematically illustrating a halftone processing unit operable using an error diffusion method according to the first exemplary embodiment of the present invention.

FIG. 21 illustrates an example of processing scan.

FIG. 22 is a flowchart illustrating an example operation that can be performed by the halftone processing unit according to the first exemplary embodiment of the present invention.

FIG. 23 illustrates example data stored in a cumulative error memory.

FIG. 24 is a flowchart illustrating example color ink quantization processing, which can be performed when a K output gradation value is 0, according to the first exemplary embodiment of the present invention.

FIG. 25 schematically illustrates an image that can be obtained though the halftone processing according to the first exemplary embodiment of the present invention.

FIG. 26 schematically illustrates a recording head and recording patterns that can be obtained by a multi-pass recording method.

FIG. 27 schematically illustrates an example dot layout on a recording medium that can be obtained through the processing according to the first exemplary embodiment of the present invention.

FIG. 28 is a block diagram illustrating a schematic image processing configuration according to a second exemplary embodiment of the present invention.

FIG. 29 is a flowchart illustrating an example operation that can be performed by a halftone processing unit according to the second exemplary embodiment of the present invention.

FIG. 30 schematically illustrates an image having been subjected to halftone processing according to the second exemplary embodiment of the present invention.

FIGS. 31A, 31B, 31C, and 31D illustrate output patterns that correspond to input levels 0 to 8, which can be obtained through conversion of a dot layout patterning processing unit.

FIG. 32 schematically illustrates an example image having been subjected to dot layout patterning processing according to the second exemplary embodiment of the present invention.

FIG. 33 schematically illustrates an example dot layout on a recording medium that can be obtained through the processing according to the second exemplary embodiment of the present invention.

FIG. 34 is a block diagram schematically illustrating an image processing configuration according to a third exemplary embodiment of the present invention.

FIG. 35 is a flowchart illustrating a processing procedure of a selection unit according to the third exemplary embodiment of the present invention.

FIG. 36 is a flowchart illustrating an example operation that can be performed by a halftone processing unit according to the third exemplary embodiment of the present invention.

FIG. 37 schematically illustrates an example pixel layout according to an exemplary embodiment of the present invention.

FIGS. 38A and 38B schematically illustrate example dither matrices that are usable in a fourth exemplary embodiment of the present invention.

FIG. 39 schematically illustrates examples of the dot layout of each color having been subjected to the halftone processing at the dark portion according to the fourth exemplary embodiment of the present invention.

FIG. 40 is a graph illustrating a plurality of areas, in which the combination of dither matrices applied to a bright portion is switched, on the a*-b* plane of the CIELab color space according to the fourth exemplary embodiment of the present invention.

FIGS. 41A, 41B, 41C, and 41D schematically illustrate example dither matrices that are usable in the fourth exemplary embodiment of the present invention.

FIG. 42 schematically illustrates examples of the dot layout of each color having been subjected to the halftone processing at the bright portion according to the fourth exemplary embodiment of the present invention.

FIG. 43 is a graph illustrating a comparison with respect to a dot overlapped state between an example subjected to the first exemplary embodiment and a random layout example.

FIG. 44 is a flowchart illustrating an example operation that can be performed by a halftone processing unit according to a fifth exemplary embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

Various exemplary embodiments, features, and aspects of the invention will be described in detail below with reference to the drawings.

In the present exemplary embodiment, recording materials are cyan, magenta, yellow, and black inks. The black ink may be referred to as a black color material. The cyan ink, the magenta ink, and the yellow ink may be collectively referred to as chromatic color materials or chromatic color inks or may be simply referred to as color inks. The black ink may be referred to as achromatic color ink. In the following description, uppercase letters C, M, Y, and K may represent respective colors or their data or hue. More specifically, “C” represents cyan color or may represent data or hue thereof. represents magenta color or may represent data or hue thereof. “Y” represents yellow color or may represent data or hue thereof. “K” represents black color or may represent data or hue thereof.

An example relationship between the spatial layout (hereinafter, referred to as “image formation”) of a color material and color development on a recording medium (e.g., a paper surface) is described in detail below with reference to attached drawings.

FIG. 2 is a graph schematically illustrating spectral reflectance characteristics of paper (as an example of the recording medium) and inks (as examples of the color material), in which the abscissa axis represents the wavelength and the ordinate axis represents the reflectance. More specifically, a spectrum 201 indicates spectral reflectance characteristics of a paper, a spectrum 202 indicates spectral reflectance characteristics of a black ink, a spectrum 203 indicates spectral reflectance characteristics of a cyan ink, and a spectrum 204 indicates spectral reflectance characteristics of a yellow ink.

The color development is described below referring to an example case where inks having the reflection characteristics illustrated in FIG. 2 are differently arranged on the paper surface as illustrated in FIG. 3. A layout 301 is a “horizontally mixed color” layout, according to which a black ink and a yellow ink are not overlapped with each other and arrayed in the horizontal direction. The paper itself is not exposed in any area of the paper surface, and the paper surface is covered entirely with either one of two inks.

The color development according to the layout 301 is dependent on a weighted average of each ink with respect to area ratio, as understood from the generally known Murray-Davis formula, and can be calculated using the following formula (1).

R(λ)=S—K×R—K(λ)+S—Y×R—Y(λ) (1)

R(λ), R_K(λ), and R_Y (λ) represent the reflectance of the mixed color, the black ink, and the yellow ink at the wavelength λ, respectively. Further, S_K and S_Y represent the area ratio (having a value of 0 to 1) of the black ink and the yellow ink on the paper surface, respectively.

Further, a layout 302 is a “color material layer” layout, according to which a black ink and a yellow ink are overlapped with each other and layered in the up-and-down direction. Similar to the layout 301, the paper itself is not exposed in any area of the paper surface, and the paper surface is covered entirely with both of two inks. The color development according to the layout 302 can be calculated using the following formula (2).

R(λ)=R—Y(λ)+{T—Y(λ)2×R—K(λ)}/{1−R—K(λ)×R—Y(λ)} (2)

R(λ), R_K(λ), and R_Y (λ) represent the reflectance of the mixed color, the black ink, and the yellow ink at the wavelength λ, respectively. Further, T_Y(λ) represents the transmittance of the yellow ink.

Especially, if the ink is a pigment ink (or any other ink including a larger light scattering component compared to a dye ink), the reflectance and the transmittance are influenced by the scattering component as well as the light absorption component. FIG. 4 schematically illustrates spectral reflectance characteristics that can be calculated about the spectral reflectance characteristics illustrated in FIG. 2, using the above-described formulae (1) and (2), in a case where the selected ink is the pigment ink (i.e., the ink including a non-negligible light scattering component as described above).

A spectrum 401 indicates spectral reflectance characteristics corresponding to the layout 301. A spectrum 402 indicates spectral reflectance characteristics corresponding to the layout 302. To simplify the comparison, the area ratios S_K and S_Y are adjusted in such a way as to equalize the spectrum 401 and the spectrum 402 in lightness, although it has no influence on the determination with respect to the steepness of the spectrum. As is understood from FIG. 4, the spectrum 401 is steep in spectral reflectance change, compared to the spectrum 402.

More specifically, the black ink having a higher light absorption rate in the entire visible wavelength range and a colored ink having a lower light absorption rate partly in the visible wavelength range (hereinafter, a colored ink other than the black ink may be referred to as “color ink”) have the following characteristics. If two inks are not overlapped with each other and arrayed in the horizontal direction (more specifically, the inks are mutually exclusive) as indicated by the layout 301, the reflectance value is smaller in a wavelength range in which the absorption by the color ink is large (i.e., a short wavelength side illustrated FIG. 4) .

On the other hand, the reflectance value is higher in a wavelength range in which the absorption by the color ink is small (i.e., a long wavelength side illustrated FIG. 4). Therefore, the spectral reflectance changes steeply as indicated by the spectrum 401. On the other hand, if two inks are mutually overlapped and layered in the up-and-down direction as indicated by the layout 302, the reflectance value is smaller because the absorption by any one of the inks is large. Accordingly, if the black ink (i.e., the ink having a higher light absorption rate in the entire visible wavelength range) is overlapped with another ink, the reflectance value is smaller in the entire wavelength range.

In a case where only the light absorption is taken into consideration, if it is presumed that the reflectance of the black ink is constant in the entire wavelength range, the spectral reflectance value of the yellow ink if it is overlapped with the black ink in the up-and-down direction is a predetermined multiple of that of the yellow ink itself. More specifically, the steep spectrum can be maintained. Therefore, the spectral reflectance characteristics do not become lower in the entire wavelength range even if the black ink is overlapped with the yellow ink.

However, as described above, the pigment ink includes a non-negligible scattering component. The absorption component, if the absorption rate thereof is high, can lower the reflectance value. The scattering component can be added as a bias to the reflectance. Further, in general, the scattering component is large in the wavelength range where the absorption is large. Therefore, the scattering component can moderate the change in the spectral reflectance at each wavelength.

FIG. 5 illustrates color development differences that may occur due to the difference between the layout 301 and the layout 302 on the L*-b* plane of the CIELab color space. The L*-b* plane illustrated in FIG. 5 is related to the yellow hue, in which the abscissa axis represents the magnitude of b* and the ordinate axis represents the magnitude of lightness L*. In FIG. 5, a point 501 indicates a chromaticity value of yellow and a point 502 indicates a chromaticity value of black. Further, a point 503 indicates a chromaticity value that corresponds to the layout 301 and a point 504 indicates a chromaticity value that corresponds to the layout 302. The saturation value becomes higher if the distance from the lightness L* axis increases. Therefore, it is understood that the saturation value of the point 503 is higher than the saturation value of the point 504. Although FIG. 5 illustrates the characteristics based on the combination of yellow and black, similar characteristics can be obtained by a combination of cyan and black or a combination of magenta and black.

The combination of inks used in the above-described example is limited to the combination of the black ink and only one colored ink. Next, a combination of a black ink and two colored inks is described below with respect to a relationship between the spatial layout of the color material layer and the color development on the paper surface.

The spectral reflectance of each ink is similar to that described in FIG. 2. The color development is described in detail below based on example layouts illustrated in FIG. 6, in which the inks having the reflection characteristics illustrated in FIG. 2 are arranged on the paper surface. As described above, it is useful that the black ink and the color ink are not overlapped with each other and arrayed in the horizontal direction to realize high saturation in color development compared to the case where the black ink and the color ink are mutually overlapped and layered in the up-and-down direction.

According to two layouts (i.e., layout 601 and layout 602) illustrated in FIG. 6, the black ink and two color inks are exclusively arranged without being overlapped with each other. The layout 601 includes a cyan ink and a yellow ink that are not overlapped with each other and arrayed in the horizontal direction. The layout 602 includes a cyan ink and a yellow ink that are mutually overlapped and layered in the up-and-down direction.

If the layout 601 is employed, the reflectance of an area where only the color ink is arranged and no black color is arranged (which may be hereinafter referred to as “color ink area”) on the paper surface can be expressed as a weighted average of the spectrum 203 and the spectrum 204 illustrated in FIG. 2 that can be calculated using the above-described formula (1). On the other hand, if the layout 602 is employed, the reflectance of the color ink area including no black ink can be calculated using the above-described formula (2) based on the spectrum 203, the spectrum 204, and transmittance (not illustrated) of each ink. FIG. 7 illustrates spectral reflectance characteristics of the color ink area in each of the layout 601 and the layout 602, which can be obtained through the above-described calculations.

A spectrum 701 indicates spectral reflectance characteristics corresponding to the layout 601. A spectrum 702 indicates spectral reflectance characteristics corresponding to the layout 602. As is understood from FIG. 7, the spectrum 702 is steep in spectral reflectance change (i.e., higher in saturation), compared to the spectrum 701. More specifically, it is understood that higher saturation can be obtained in color development when the color inks are mutually overlapped and layered in the up-and-down direction compared to the case where the color inks are not overlapped with each other and arrayed in the horizontal.

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